IMS and MS Dynamic Regimes

IMS is within a limited range for a great diversity of ions, varying by just one order of magnitude between small atomic species and proteins (3.2.1). Nonetheless, increase of frx for extremely large macroions and nanoparticles eventually constrains the mass range of differential IMS instruments (3.2.1). 

These formulae clearly show the decisive role of gas pressure in delineating the boundary between IMS and MS regimes. To estimate the magnitude of P needed for IMS operation, let us use a hypothetical medium-size ion (m = 1000 Da, z = 1) with Xo = 1 cm /(V s) that is typical for such species in N2 or air. Assuming a device of moderate size (L = 0.1 m), we find 0.1 Torr for a moderate field of = lO V/m and P 0.01 Torr for a weak field of = 100 V/m. The minimum P values would decrease by 3 times for L = 1 m, which is close to maximum dimensions for reasonable instruments. The mobilities of ions in He normally exceed those in N2 by 3—4 times, because of (i) in Equation 1.10 increasing (for ions 

The pressure boundary between MS and IMS regimes depends on the ion via m, z, and Ko in Equation 1.17 as P oc Koy/mjz, but in real cases the effects of those variables largely cancel. Substituting oc z/fi and ft oc we find that P scales as Hence, for singly charged ions, the lower pressure limit for IMS drops 

---